1. Field of the Invention
The present invention relates to a laser apparatus having an optical resonator provided with a reflection portion such as a semiconductor laser.
2. Description of the Related Art
A semiconductor laser for example has an optical resonator constituted by a pair of opposing reflection mirrors, that is, a Fabry-Perot resonator.
It has been known that when a semiconductor laser having such a structure is built in an optical system with return light, the optical output thereof changes along with a change of the phase of the return light (refer to for example Ryoichi Ito and Michiharu Nakamura ed., Semiconductor Laserxe2x80x94Fundamentals and Applicationsxe2x80x94, Baifukan, 1989).
This fluctuation of the optical output becomes harmful noise in various applications of semiconductor lasers.
For example, it becomes a cause of noise in the recording or reproduction signal in the case of recording or reproduction from an optical disk, becomes a cause of noise in an optical signal in the case of optical communication, and becomes a cause of uneven color density or uneven color in the case of a laser beam printer.
Therefore, in the related art, generation of noise has been suppressed by installing an optical isolator, an optical shield plate, or a prism in the optical system so as to prevent the return light from striking the optical resonator of the semiconductor laser (refer to for example Japanese Unexamined Patent Publication (Kokai) No. 61-151849, Japanese Unexamined Patent Publication (Kokai) No. 3-54730, and Japanese Unexamined Patent Publication (Kokai) No. 6-162551), arranging an attenuation plate etc. for reducing the return light in comparison with the output of the semiconductor laser (refer to for example Japanese Unexamined Patent Publication (Kokai) No. 62-137734) and suppressing the influence of the return light, or superimposing a high frequency on a drive current particularly in the recording and reproduction from an optical disk (refer to for example Japanese Unexamined Patent Publication (Kokai) No. 60-140551).
However, in these techniques of the related art mentioned above, there are the disadvantages that the number of parts becomes large, the entire apparatus becomes bigger, the assembly or adjustment thereof is troublesome, and the power consumption is increased.
The present invention was made in consideration with such a circumstance and has as an object thereof to provide a laser apparatus capable of suppressing the influence of return light on the output light without causing an increase of the number of parts, an enlargement of the entire apparatus, or an increase of the power consumption.
To attain the above object, according to a first aspect of the present invention, there is provided a laser apparatus having an optical resonator provided with a reflection portion and obtaining a constant optical output from the optical resonator under constant drive conditions, wherein a reflectivity of the reflection portion is set so as to absorb a change of an intensity of an output light due to a change of a phase of return light.
Preferably, the laser apparatus is a current-driven semiconductor laser, and the reflectivity of the reflection portion is set so that there is a predetermined optical output value in a region in which current-optical output characteristics expressing the optical output with respect to the drive current in different return light phases intersect.
According to a second aspect of the present invention, there is provided a laser apparatus having an optical resonator provided with a pair of opposing reflection mirrors, obtaining a constant optical output from said optical resonator under constant drive conditions, and used built-in an optical system in which part RofbP (0 less than Rofb less than 1) of the optical output P returns to the optical resonator as return light, wherein
whereby a time average value of the optical output P is defined as Pav, where an absolute value |xcex94P| of a fluctuation xcex94P=Pxe2x88x92Pav of the optical output produced along with the change of a return light phase Ø must not be more than a constant proportion xcex5 with respect to Pav and where |xcex94P|/Pav must be not more than xcex5,
whereby the following are defined by designating a reflectivity of the side (forward) for fetching the optical output of the pair of opposing reflection mirrors constituting said optical resonator as Rf, the reflectivity of the side opposite to this (reverse) as Rr, a resonator length as Lin, an internal loss as xcex11, a differential gain as gN, a proportion of coupling of the return light with the light inside the laser as xcex7t, the proportion of coupling of the return light with the output light from the laser as xcex7r, and an input/output efficiency and a threshold gain measured by a laser apparatus element where there is no return light as xcex7s0 and g0, respectively, and using these parameters or physical constants:             Y      1        =          1      +                                    η            r                    ⁢                      R            ofb                    ⁢                      R            f                                          Y      2        =          1      -                                    η            r                    ⁢                      R            ofb                    ⁢                      R            f                                          Z      1        =          1      +                                                                  η                t                            ⁢                              R                ofb                                                    R              f                                      ⁢                              (                          1              -                              R                f                                      )                                Y            1                                          Z      2        =          1      -                                                                  η                t                            ⁢                              R                ofb                                                    R              f                                      ⁢                              (                          1              -                              R                f                                      )                                Y            2                                          g      1        =                  g        0            -                        1                      L            in                          ⁢        ln        ⁢                  xe2x80x83                ⁢                  Z          1                                g      2        =                  g        0            -                        1                      L            in                          ⁢        ln        ⁢                  xe2x80x83                ⁢                  Z          2                                xe2x80x83        ⁢                            η          S1                          η          S          0                    =                                                  g              0                        ⁢                          (                                                g                  1                                -                                  α                  i                                            )                                ⁢                      (                          1              -                                                                    R                    f                                    ⁢                                      R                    r                                                                        )                    ⁢                      (                          1              +                                                                    R                    f                                    /                                      R                    r                                                                        )                                                              g              1                        ⁢                          (                                                g                  0                                -                                  α                  i                                            )                                ⁢                                    Y              1              2                        ⁢                          (                              1                -                                                                                                    R                        f                                            ⁢                                              R                        r                                                                              ⁢                                      Z                    1                                                              )                                ⁢                      (                          1              +                                                                                          R                      f                                        /                                          R                      r                                                                      ⁢                                  Z                  1                                                      )                                                  η        S2                    η        S        0              =                                        g            0                    ⁢                      (                                          g                2                            -                              α                i                                      )                          ⁢                  (                      1            -                                                            R                  f                                ⁢                                  R                  r                                                              )                ⁢                  (                      1            +                                                            R                  f                                /                                  R                  r                                                              )                                                  g            2                    ⁢                      (                                          g                0                            -                              α                i                                      )                          ⁢                              Y            2            2                    ⁢                      (                          1              -                                                                                          R                      f                                        ⁢                                          R                      r                                                                      ⁢                                  Z                  2                                                      )                          ⁢                  (                      1            +                                                                                R                    f                                    /                                      R                    r                                                              ⁢                              Z                2                                              )                    
the following stands:                               2          ⁢          ε                ≥                              "LeftBracketingBar"                                          (                                                                            η                      S1                                                              η                      S                      0                                                        -                                                            η                      S2                                                              η                      S                      0                                                                      )                            +                                                                    η                    S                    0                                                                              P                      av                                        ⁢                                          g                      N                                        ⁢                                          L                      in                                                                      ⁡                                  [                                                                                                              η                          s1                                                                          η                          S                          0                                                                    ⁢                      ln                      ⁢                                              xe2x80x83                                            ⁢                                              Z                        1                                                              -                                                                                            η                          S2                                                                          η                          S                          0                                                                    ⁢                      ln                      ⁢                                              xe2x80x83                                            ⁢                                              Z                        2                                                                              ]                                                      "RightBracketingBar"                    .                                    (        A        )            
According to a third aspect of the present invention, there is provided a semiconductor laser apparatus having the optical resonator provided with a pair of opposing reflection mirrors, obtaining a constant optical output from said optical resonator under constant drive conditions, and used built-in an optical system in which part RofbP (0 less than Rofb less than 1) of the optical output P returns to the optical resonator as return light, wherein
whereby a time average value of the optical output P is defined as Pav, where an absolute value |xcex94P| of the fluctuation xcex94P=Pxe2x88x92Pav of the optical output produced along with a change of a return light phase Ø must not be more than a constant proportion xcex5 with respect to Pav and where |xcex94P|/Pav must be not more than xcex5,
whereby the following are defined by designating a reflectivity of the side (forward) for fetching the optical output of the pair of opposing reflection mirrors constituting said optical resonator as Rf, a reflectivity of the side opposite to this (reverse) as Rr, the resonator length as Lin, the internal loss as xcex11, a volume of an active region as Va, an optical confinement coefficient of the active region as xcex93, a proportion of an increase of the optical gain with respect to an increase of a carrier density of the active region as gN, a carrier life of the carrier of the active region by spontaneous emission and a nonradiative recombination as xcfx84s, a proportion of coupling of the return light with the light inside the laser as xcex7t, a proportion of coupling of the return light with an output light from the laser as xcex7r, a slope efficiency and the threshold gain measured by a laser apparatus element where there is no return light as xcex7s0 and g0, respectively, and an amount of charge as e, respectively, and using these parameters or physical constants:             Y      1        =          1      +                                    η            r                    ⁢                      R                          ofb              ⁢                              xe2x80x83                                              ⁢                      R            f                                          Y      2        =          1      -                                    η            r                    ⁢                      R            ofb                    ⁢                      R            f                                          Z      1        =          1      +                                                                  η                t                            ⁢                              R                ofb                                                    R              f                                      ⁢                              (                          1              -                              R                f                                      )                                Y            1                                          Z      2        =          1      -                                                                  η                t                            ⁢                              R                ofb                                                    R              f                                      ⁢                              (                          1              -                              R                f                                      )                                Y            2                                          g      1        =                  g        0            -                        1                      L            in                          ⁢        ln        ⁢                  xe2x80x83                ⁢                  Z          1                                g      2        =                  g        0            -                        1                      L            in                          ⁢        ln        ⁢                  xe2x80x83                ⁢                  Z          2                                xe2x80x83        ⁢                            η          S1                          η          S          0                    =                                                  g              0                        ⁢                          (                                                g                  1                                -                                  α                  i                                            )                                ⁢                      (                          1              -                                                                    R                    f                                    ⁢                                      R                    r                                                                        )                    ⁢                      (                          1              +                                                                    R                    f                                    /                                      R                    r                                                                        )                                                              g              1                        ⁢                          (                                                g                  0                                -                                  α                  i                                            )                                ⁢                                    Y              1              2                        ⁢                          (                              1                -                                                                                                    R                        f                                            ⁢                                              R                        r                                                                              ⁢                                      Z                    1                                                              )                                ⁢                      (                          1              +                                                                                          R                      f                                        /                                          R                      r                                                                      ⁢                Z                ⁢                                  xe2x80x83                                ⁢                sub1                                      )                                          xe2x80x83        ⁢                            η          S2                          η          S          0                    =                                                  g              0                        ⁢                          (                                                g                  2                                -                                  α                  i                                            )                                ⁢                      (                          1              -                                                                    R                    f                                    ⁢                                      R                    r                                                                        )                    ⁢                      (                          1              +                                                                    R                    f                                    /                                      R                    r                                                                        )                                                              g              2                        ⁢                          (                                                g                  0                                -                                  α                  i                                            )                                ⁢                                    Y              2              2                        ⁢                          (                              1                -                                                                                                    R                        f                                            ⁢                                              R                        r                                                                              ⁢                                      Z                    2                                                              )                                ⁢                      (                          1              +                                                                                          R                      f                                        /                                          R                      r                                                                      ⁢                                  Z                  2                                                      )                              
the following stands:                               2          ⁢          ε                ≥                  "LeftBracketingBar"                                    (                                                                    η                    S1                                                        η                    S                    0                                                  -                                                      η                    S2                                                        η                    S                    0                                                              )                        +                                                            e                  ⁢                                      xe2x80x83                                    ⁢                  V                  ⁢                                      xe2x80x83                                    ⁢                  a                  ⁢                                      xe2x80x83                                    ⁢                                      η                    S                    0                                                                                        P                    av                                    ⁢                  Γ                  ⁢                                      xe2x80x83                                    ⁢                                      g                    N                                    ⁢                                      L                    in                                    ⁢                                      τ                    S                                                              ⁡                              [                                                                                                    η                        s1                                                                    η                        S                        0                                                              ⁢                    ln                    ⁢                                          xe2x80x83                                        ⁢                                          Z                      1                                                        -                                                                                    η                        S2                                                                    η                        S                        0                                                              ⁢                    ln                    ⁢                                          xe2x80x83                                        ⁢                                          Z                      2                                                                      ]                                              "RightBracketingBar"                                    (        B        )            
Preferably, when there is return light, it operates where the fluctuation in the optical output due to the return light in a specific optical output is not more than a constant proportion (xcex5).